12 replies to this topic

[jt19]

Hi

 

I'm going to completely flush my brake system out.

 

Would I benefit from re filling up with dot 5?

 

It doesn't absorb water, has a higher boiling point than dot 4, it's not harmful to paint work.

 

Why is it not used more? What are the cons with using dot 5?

 

 

Thanks

Josh 

[benjy_18]

Dot 5 is silicone based iirc, unsuitable for a lot of master cylinders due to it eating the seals.

Stick to dot 3/4 and just change regularly

Edited by benjy_18, 12 February 2014 - 08:25 PM.

[Mini Manannán]

DOT 4 is what you want, as said above, DOT 5 is unsuitable for your mini.

Edited by Mini Manannán, 12 February 2014 - 08:28 PM.

[jt19]

ok thanks guys 

[gazza82]

IIRC DOT 5,1 is fine in DOT 3/4 systens but you need to flush it right through.

If concerned try Castrol Response whuch is DOT4 but synthetic.Again flush all old fluid out. Been using this in my Alfa for several years with no issues.

[dklawson]

There is some minor misinformation above.

 

DOT-5.1 is not the same as DOT-5.  DOT-5.1 is more like a traditional glycol based brake fluid.  DOT-5 is silicone based as mentioned above.

 

DOT-5 does NOT eat seals.  DOT-5 is not mixable with DOT-3/4/5,1 and therefore is typically used when all the rubber bits have been replaced... but NOT because it eats the seals.  HOWEVER, when the seals are old and have been used with DOT-3 or -4, the old seals will sluff off a surface layer of rubber which will turn your new DOT-5 fluid black.  DOT-5 does have the reputation of damaging the hydraulic brake pressure switches used on early cars.  Obviously this has no impact on later cars that use pedal actuated brake light switches.

 

Why is DOT-5 not used more often?  There are several reasons.  I will list a few but you will find others on the net.

• DOT-5 traps air bubbles easier which can lead to a spongy brake pedal if precautions are not taken during filling and bleeding.
• DOT-5 can be "squishy" compared to glycol based fluids.  Again, a spongy pedal.  Using braided brake hoses helps restore some (but not all) pedal firmness.
• DOT-5 has less lubricity than glycol fluids.  This can result in seals that weep a bit compared to glycol fluid.
• DOT-5 can be abused.  The fact that it does not absorb moisture can make people NOT change it periodically.  Like all fluids, it does need periodic replacement.
• DOT-5 is silicone based.  While it won't damage paint, if spilled on paint, touch-up or repair of the paint can be complicated.
• DOT-5 is not allowed by most racing organizations.

 

Why to use DOT-5?

• It doesn't absorb moisture like glycol based fluids.  This makes it ideal for collector cars that are not driven frequently or for whom the brake rebuild parts (wheel and master cylinders) are hard to come by or expensive.
• It does not attack paint.  Ideal if you have done a thorough rebuild of a collector car.

 

When you search for information on DOT-5 you will find it discussed at length on forums involving collector cars.  In general you will find that DOT-5 is not used in daily drivers or performance cars but is used extensively in collector cars that are used sparingly.

 

As I mentioned above, if you decide to use DOT-5, the change is typically made when all the rubber bits are replaced.  That includes master cylinder seals, wheel cylinder/caliper seals, rubber flex hoses, the PDWA seals (when fitted), and proportioning valve seals.  It is common to flush the hard metal lines with isopropyl alcohol and then run compressed air through the system to flush out the alcohol and any moisture.  You can easily see that switching to DOT-5 is not typically justified on a daily driver.

[ado15]

...........DOT-5 does NOT eat seals. ...............

 

Sorry.... double post for some reason!

Edited by ado15, 13 February 2014 - 10:34 AM.

[ado15]

...........DOT-5 does NOT eat seals. ...............

 

Doug, we are aware of a seal manufacturer that won't guarantee their seals with DOT5 fluid.

 

As an aside, my only personal experience of silicone based fluid is that I could never get a firm pedal. It always felt a little spongy when compared to conventional fluids. This obviously backs up your comments on the subject.

Edited by ado15, 13 February 2014 - 10:32 AM.

[Ivor Badger]

This begs the question, how long does it take for the fluid to absorb enough water to make a difference and actually cause problems?

[sonikk4]

Most manufacturers recommend replacing your brake fluid every two years and this sounds like a sensible amount of time.

Other factors do come in to it though but these should not really gave a detrimental effect. I'm talking overheating of brake fluid caused by excessive use of the brakes. Something that really should not happen on a daily driver unless of course you use your car as a track day vehicle as well.

If you don't know when the brake fluid was changed then think about doing it sooner rather than later. And I would stick with DOT 4.

[tiger99]

I used DOT 5 in several high mileage cars, after a full brake system overhaul, replacing all seals, hoses (Goodridge, cheaper than OEM in some cases) and pipes (Kunifer of course, NEVER copper), and they went on to achieve very high mileages, up to 200k, without further attention to the brakes, except routine pad, and occasionally disc, changes. The fluid remained clean, and did not need to be changed.

 

However, none of these cars was a Mini, and although I have had 3 Minis and must have flushed and bled the brake systems many times, including full seal and hose replacements on 2 of them, and never had the slightest difficulty in bleeding any of them, there are many sad tales on this forum of people who can't get them to bleed (may depend on brake system configuration), and for that reason I would suggest that the use of DOT 5 in a Mini is strictly for experts, or those with a reliable method, such as a suction bleeder, of getting the air out. The rubber hose in jar technique never works properly, and certainly will not with DOT 5.

 

If you are worried about brake temperature, and the risk of sudden failure caused by boiling fluid, upgrade to DOT 5.1, but note that it absorbs water faster than DOT 4, and should be changed more frequently. DOT 4 should always be changed every 2 years or less, and it is very clear from this forum that there are a great many Minis on the road where that is never done, and are seriously risking brake failure when it boils, as well as accelerated corrosion of cylinders. Numerous clutch problems are also caused by neglect of the hydraulics, so do it at the same time. It is cheaper, as well as being much safer, to rigorously adhere to the recommended fluid change interval, as you will get longer life from seals and cylinders. And, never, ever do anything that carries the slightest risk of getting traces of mineral oil in the system, such as using penetrating oil on hydraulic unions or bleed nipples, as it only needs a trace to destroy all the seals in the system.

[Cooperman]

I use DOT 4 in road cars and AP600 competition fluid in rally cars to avoid the fluid boiling, which DOT 4 can and does do under extreme conditions.

Does anyone remember the old 'Girling Amber' high boiling point fluid? AP 600 is about like that.

You can buy AP 600 from Demon Tweeks via their web site, but don't mix it with DOT 4. It is, however, DOT 4 compatible so far as I'm aware and you simply pump out the DOT 4 and re-fill with AP 600.

[dklawson]

 

...........DOT-5 does NOT eat seals. ...............

 

Doug, we are aware of a seal manufacturer that won't guarantee their seals with DOT5 fluid.

 

As an aside, my only personal experience of silicone based fluid is that I could never get a firm pedal. It always felt a little spongy when compared to conventional fluids. This obviously backs up your comments on the subject.

 

I have seen the same warranty statements for new TRW manufactured cylinders and similar warnings from two shops that sleeve old cylinders for antique cars.  When I inquired about "why", in both cases I was told it had to do with the lubricity of the DOT-5 fluid preventing the seals from operating as they do with DOT-3 and -4.  The firms took the position that DOT-5 required different surface finishes in the bore to work properly.  When your product involves safety items you certainly would want to protect yourself from any liability issues by not endorsing the use of alternate brake fluid.

 

That said, I have DOT-5 in my Mini and our Triumph GT6.  They are weekend cars that see little use.  However, when recently helping a friend rebuild the brakes in his Mini I insisted he buy a quart of DOT-4 fluid, that we replace any suspect component, and obviously flush out all the old fluid since he had no idea about the car's service history.  As it turned out, the only old bits of the original braking system left were a handful of non-corroded hard pipes and the master cylinder.  

 

As Tiger said, DOT-4 fluid does need frequent changes to make sure moisture buildup in the fluid does not cause problems.  However, you CAN bleed DOT-5 yourself using normal methods when extra care is taken.  For example, to avoid introducing bubbles you can pour the fluid into the MC reservoir by letting it run down the shaft of a clean screwdriver rather than pouring directly.  Gravity bleeding to fill the lines can minimize churning air into the system.  Final bleeding into a catch jar using the 2-person method will work when the team work well together.  If at any point you feel you have introduced bubbles into the fluid the general recommendation given is to let the car sit for several hours or overnight to allow the bubbles to come out of suspension.  Again... DOT-5 is not for everyone and is really aimed at antique and collector cars, not daily drivers.